The present invention relates generally to devices for measuring very small amounts of movement and, more particularly, to a device which is capable of measuring mechanical strain and converting this measurement into an indication of applied force.
In injection molding, die casting, or the like arts, it is generally desirable to know the amount of clamping force being applied to the die pair which makes up the mold. Aside from the fact that such knowledge will help to avoid excessive force from being placed onto the dies, the amount of clamping force may also affect the quality of the parts being formed in the mold.
Generally, machines used for injection molding, die casting, or the like have a four tie bar design, wherein each bar is positioned near a corner of a square, movable platen. A suitable clamping device is also provided to move this platen toward a stationary platen. As the platens, which contain the dies, are forced together to form a mold, the tie bars stretch in a substantially linear fashion. In this regard, manufacturers of such machines typically provide the user with information on expected tie bar stretch or deflection parameters. Generally speaking, elastic tie bar deflection will range from about 22 to about 25 one-thousandths of an inch for a molding machine capable of producing a clamping force of 200 tons.
Thus, there is a need in the art to accurately determine the strain in such tie bars, and convert this strain information into a force measurement for display to the operator or for transmission to a machine controller. By having reliable information on the force exerted on the dies, the operator or controller can assure that the mold will repeatedly produce consistent parts over many cycles, even if the mold is placed in another molding machine.
Some manufacturers do offer optional pressure sensors that can be incorporated into their injection molding machines to sense the amount of hydraulic force exerted on the die pair during the clamping and lock up condition of the platens. However, this type of sensor mechanism is somewhat difficult to utilize due to the fact that the operator must precisely record the hydraulic pressure at the very instant the lock up condition occurs to obtain an accurate reading of the force applied on the dies. Additionally, this mechanism is relatively expensive and is very sensitive to fluctuations in hydraulic pressure.
Another type of sensor mechanism used in the art includes a pair of magnetic bases associated with a tie bar and a dial indicator. As the tie bar stretches, the magnets move apart and a gauge, which is set in tons, determines the force exerted on the dies. However, the accuracy of this type of device is limited, and it must be carefully calibrated each time that it is used on different types of machines.
Accordingly, it is a principal objective of the present invention to provide a device which is capable of measuring very small amounts of relative movement, such as would result from the strain on an elongated member, and converting this measurement into an indication of a function related to this movement.
It is a more specific object of the present invention to provide a device which is capable of measuring linear movement resulting from strain on a metallic tie rod, and converting this measurement into an indication of the force being placed on this tie rod.
It is another object of the present invention to provide a strain measurement device which is capable of amplifying the movement resulting from the strain in order to obtain a more accurate reading.
It is an additional object of the present invention to provide a strain measurement device which is inexpensive, easy to calibrate and easy to install.
It is a further object of the present invention to provide a reliable strain measurement device which will generate a signal which can be used both to display an indication of force and provide an appropriate signal for input to a variety of process control systems.
To achieve the foregoing objectives, the present invention provides a unique strain measurement device which will mechanically amplify small relative movements, such as would result from the strain in a tie bar of a molding machine. In general, the device comprises an elongated member which will pivot in response to the stretching movement of the tie bar, a sensing unit for measuring the movement of the elongated member and a circuit for determining the amount of force from the relative movement measured by the sensing unit.
From the subsequent detailed description of the preferred embodiment and appended claims taken in conjunction with the accompanying drawings, other objects and advantages of the present invention will become apparent to one skilled in the art.